Turbomachine blade and relative production method

ABSTRACT

A turbomachine blade having a metal coupling root, and a metal airfoil-shaped oblong member cantilevered projecting from the coupling root; the airfoil-shaped oblong member being divided into: a lower connecting fin cantilevered projecting from and formed in one piece with the coupling root; an upper connecting fin cantilevered projecting from a coupling head towards the coupling root and formed in one piece with the coupling head; and a main plate-like body which is shaped and positioned between the two connecting fins to form an extension of the fins, and is butt-welded to the same connecting fins to form one piece with the fins.

BACKGROUND Technical Field

The present disclosure relates to a turbomachine blade and related production method.

More specifically, the present disclosure relates to a method of producing a rotor or stator blade for a compressor or front fan of an aircraft turbine engine, to which use the following description refers purely by way of example.

Description of the Related Art

As is known, rotor and stator blades of aircraft turbine engine compressors substantially comprise a coupling root designed to fit and lock rigidly to the compressor center hub or blade mounting disk; and an airfoil-shaped oblong member, which extends from the coupling root, so as to cantilever radially outwards of the hub or blade mounting disk when the coupling root is fixed inside the center hub or blade mounting disk. Stator blades also have an upper coupling head, which is located at the top end of the airfoil-shaped oblong member, i.e. at the opposite end to the coupling root, and is designed to fit and lock rigidly to the outer blade mounting ring of the compressor.

Having to withstand fairly severe mechanical stress and heat, the lower coupling root, the airfoil-shaped oblong member, and the upper coupling head (if there is one) are usually formed in one piece from a single block of high-strength metal, which is variously machined to shape the blade as required.

Milling a blade from a single block of metal, however, is an extremely painstaking, time-consuming job requiring special numeric-control milling machines capable of machining large workpieces to a high degree of precision. As a result, blades of this sort are extremely expensive to produce.

BRIEF SUMMARY DISCLOSURE

It is therefore an aim of the present disclosure to provide a method of producing a turbomachine blade which can significantly reduce the cost of manufacturing metal blades for compressors of aircraft turbine engines.

In compliance with the above aims, according to the present disclosure there is provided a turbomachine blade as defined in claim 1 and preferably, though not necessarily, in any one of the claims dependent on claim 1.

According to the present disclosure, there is also provided a method of producing a turbomachine blade, as defined in claim 10 and preferably, though not necessarily, in any one of the claims dependent on claim 10.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

A non-limiting embodiment of the present disclosure will be described by way of example with reference to the attached drawings, in which:

FIG. 1 shows a perspective view of a turbomachine blade in accordance with the teachings of the present disclosure;

FIG. 2 shows a sectioned perspective view of the FIG. 1 blade;

FIGS. 3 to 12 schematically show some steps in the method of producing the FIGS. 1 and 2 turbomachine blade; whereas

FIG. 13 shows a partly exploded perspective view of a second embodiment of the FIG. 1 turbomachine blade.

DETAILED DESCRIPTION DISCLOSURE

With reference to FIGS. 1 and 2, number 1 indicates as a whole a turbomachine blade, which may be advantageously used, for example, in the compressor or front fan of a turbine engine preferably, though not necessarily, of the type used in aircraft. Blade 1 is made of metal, and substantially comprises: a lower coupling root 2 designed to fit and lock rigidly to the turbine engine hub or center blade mounting disk (not shown); an airfoil-shaped oblong member 3 which cantilevered extends from coupling root 2, so as to cantilevered project substantially radially outwards of the hub or blade mounting disk (not shown) when coupling root 2 is fixed inside the hub or center blade mounting disk; and an upper coupling head 4, which is located at the second end of airfoil-shaped oblong member 3, i.e. at the opposite end to coupling root 2, and is designed to fit and lock rigidly to the turbine engine outer blade mounting ring (not shown).

In the example shown, coupling root 2, airfoil-shaped oblong member 3, and coupling head 4 are preferably, though not necessarily, made of titanium alloy, aluminum alloy, or special high-strength steel.

In addition, airfoil-shaped oblong member 3 is divided into a lower connecting fin 5, which cantilevered projects from coupling root 2 towards coupling head 4 and is formed in one piece with coupling root 2; an upper connecting fin 6, which cantilevered projects from coupling head 4 towards coupling root 2 and is formed in one piece with coupling head 4; and a center plate-like body 7, which is shaped and positioned between the two connecting fins 5 and 6 to form an extension of the fins, and is butt-welded to connecting fins 5 and 6 to form one piece with connecting fins 5 and 6.

More specifically, the lower edge 7 a of center plate-like body 7 and the ridge 5 a of lower connecting fin 5 are shaped to mate along substantially strip-shaped first conjugate mating surfaces A₅, A₇ complementary to each other, and which extend seamlessly from the leading edge 3 a to the trailing edge 3 b of airfoil-shaped oblong member 3 along a curved profile/path in the laying plane/centerline plane P of airfoil-shaped oblong member 3. At least along a center portion of a length preferably equal to over 60% of the total length of conjugate mating surfaces A₅, A₇, conjugate mating surfaces A₅, A₇ are preferably of substantially constant width l_(a) substantially equal locally to the maximum thickness of center plate-like body 7.

In other words, the lower edge 7 a of center plate-like body 7 is defined by strip-shaped mating surface A₇, the ridge 5 a of lower connecting fin 5 is defined by strip-shaped mating surface A₅ perfectly complementary to strip-shaped mating surface A₇, and the two strip-shaped mating surfaces A₅ and A₇ mate one to the other.

More specifically, the width l_(a) of strip-shaped mating surfaces A₅ and A₇ is substantially equal to the maximum thickness of center plate-like body 7 along a center portion of a length equal to over 80% of the total length of strip-shaped mating surfaces A₅ and A₇.

More specifically, in the example shown, the width l_(a) of strip-shaped mating surfaces A₅ and A₇ is preferably substantially equal to the maximum thickness of center plate-like body 7 up to a distance of less than 10 mm (millimeters) from leading edge 3 a and trailing edge 3 b of airfoil-shaped oblong member 3 respectively.

Moreover, strip-shaped mating surfaces A₅ and A₇ preferably, though not necessarily, extend from leading edge 3 a to trailing edge 3 b of airfoil-shaped oblong member 3 along a curved, substantially Ω (omega) shaped profile/path.

Likewise, the upper edge 7 b of center plate-like body 7 and the ridge 6 b of upper connecting fin 6 are shaped to mate along substantially strip-shaped second conjugate mating surfaces B₆, B₇ complementary to each other, and which extend seamlessly from the leading edge 3 a to the trailing edge 3 b of airfoil-shaped oblong member 3 along a curved profile/path in the laying plane/centerline plane P of airfoil-shaped oblong member 3. At least along a center portion of a length preferably equal to over 60% of the total length of conjugate mating surfaces B₆, B₇, second conjugate mating surfaces B₆, B₇ are preferably of substantially constant width l_(b) substantially equal to the maximum thickness of center plate-like body 7.

Obviously, the width l_(b) of strip-shaped mating surfaces B₆ and B₇ also substantially equals width l_(a) of strip-shaped mating surfaces A₅ and A₇.

In other words, the upper edge 7 b of center plate-like body 7 is defined by strip-shaped mating surface B₇, the ridge 6 b of upper connecting fin 6 is defined by strip-shaped mating surface B₆ perfectly complementary to strip-shaped mating surface B₇, and the two strip-shaped mating surfaces B₆ and B₇ mate one to the other.

More specifically, the width l_(b) of strip-shaped mating surfaces B₆ and B₇ is substantially equal to the maximum thickness of center plate-like body 7 along a center portion of a length equal to over 80% of the total length of strip-shaped mating surfaces B₆ and B₇.

More specifically, in the example shown, the width l_(b) of strip-shaped mating surfaces B₆ and B₇ is preferably substantially equal to the maximum thickness of center plate-like body 7 up to a distance of less than 10 mm (millimeters) from leading edge 3 a and trailing edge 3 b of airfoil-shaped oblong member 3 respectively.

Moreover, second strip-shaped mating surfaces B₆ and B₇ preferably, though not necessarily, extend from leading edge 3 a to trailing edge 3 b of airfoil-shaped oblong member 3 along a curved, substantially Ω (omega) shaped profile/path. With reference to FIGS. 3 and 4, the method of producing blade 1 comprises:

making, preferably by milling and/or stamping and/or forging and/or die-casting, a first semifinished metal part 20, the three-dimensional shape of which over-approximates the shape of coupling root 2 of blade 1, and which also has a projecting appendage 25, the three-dimensional shape of which over-approximates the shape of lower connecting fin 5;

making, preferably by stamping and/or bending and/or forging and/or die-casting, a metal plate 70 of substantially constant thickness s over-approximating the maximum thickness of center plate-like body 7 of airfoil-shaped oblong member 3 of blade 1, and the three-dimensional shape of which over-approximates the shape of center plate-like body 7 of airfoil-shaped oblong member 3; and

making, preferably by milling and/or stamping and/or forging and/or die-casting, a second semifinished metal part 40, the three-dimensional shape of which over-approximates the shape of coupling head 4 of blade 1, and which also has a projecting appendage 46, the three-dimensional shape of which over-approximates the shape of upper connecting fin 6. As used herein, “over-approximates” and “over-approximating” refers to approximating with a larger value or size.

More specifically, the difference between the thickness s of metal plate 70 and the maximum thickness of center plate-like body 7 is less than 2 millimeters, and preferably even less than 1 millimeter.

In the example shown, in particular, the thickness s of metal plate 70 preferably ranges between 5 mm and 40 mm (millimeters).

Again with reference to FIGS. 3 and 4, the method of producing blade 1 also comprise:

-   -   removing, preferably by cutting and subsequent milling and/or         other material-removing machining operation, the bottom portion         71 of plate 70 located beyond a first transverse reference line         T_(a) having a curved profile in the laying plane/centreline         plane P of plate 70; and

removing, preferably by cutting and/or milling and/or other material-removing machining operation, the portion of the ridge 25 a of projecting appendage 25 of semifinished part 20 located beyond a second transverse reference line F_(a) which, in the laying plane/centreline plane P of projecting appendage 25, has a curved profile substantially identical to that of first transverse reference line T_(a).

Preferably, first transverse reference line T_(a) is the locus of points in which the thickness of plate 70 over-approximates locally the maximum thickness of centre plate-like body 7 of airfoil-shaped oblong member 3 of blade 1, preferably by a margin of less than a millimeter. And likewise, second transverse reference line F_(a) is the locus of points in which the thickness of projecting appendage 25 over-approximates locally the maximum thickness of center plate-like body 7 of airfoil-shaped oblong member 3 of blade 1, preferably by a margin of less than a millimeter.

Preferably, first transverse reference line T_(a) has a curved, substantially Ω (omega) shaped profile; and second transverse reference line F_(a) has a curved, substantially Ω (omega) shaped profile substantially identical to that of transverse reference line T_(a).

Next, as shown in FIGS. 5 and 6, the method of producing blade 1 comprises:

placing the lower edge 70 a, corresponding to first transverse reference line T_(a), of plate 70 on the ridge 25 a, corresponding to second transverse reference line F_(a), of projecting appendage 25 of semifinished part 20, so that plate 70 is aligned locally with appendage 25 of semifinished part 20; and

butt-welding the lower edge 70 a of plate 70 to projecting appendage 25 of semifinished part 20 to connect the two parts rigidly to each other.

As shown in FIGS. 3 and 4, the method of producing blade 1 also comprises:

removing, preferably by cutting and subsequent milling and/or other material-removing machining operation, the top portion 72 of plate 70 located beyond a third transverse reference line T_(b) having a curved profile in the laying plane/centerline plane P of plate 70; and

removing, preferably by cutting and/or milling and/or other material-removing machining operation, the portion of ridge 46 b of projecting appendage 46 of semifinished part 40 located beyond a fourth transverse reference line F_(b) which, in the laying plane/centerline plane P of projecting appendage 46, has a curved profile substantially identical to that of third transverse reference line T_(b).

Preferably, third transverse reference line T_(b) is the locus of points in which the thickness of plate 70 over-approximates locally the maximum thickness of center plate-like body 7 of airfoil-shaped oblong member 3 of blade 1, preferably by a margin of less than a millimeter. And likewise, fourth transverse reference line F_(b) is the locus of points in which the thickness of projecting appendage 46 over-approximates locally the maximum thickness of center plate-like body 7 of airfoil-shaped oblong member 3 of blade 1, preferably by a margin of less than a millimeter.

Preferably, third transverse reference line T_(b) has a curved, substantially Ω (omega) shaped profile; and fourth transverse reference line F_(b) has a curved, substantially Ω (omega) shaped profile substantially identical to that of transverse reference line T_(b).

Next, as shown in FIGS. 5 and 7, the method of producing blade 1 comprises:

placing the upper edge 70 b, corresponding to third transverse reference line T_(b), of plate 70 on the ridge 46 b, corresponding to fourth transverse reference line F_(b), of projecting appendage 46 of semifinished part 40, so that plate 70 is aligned locally with projecting appendage 46 of semifinished part 40; and

butt-welding the upper edge 70 b of plate 70 to projecting appendage 46 of semifinished part 40 to connect the two parts rigidly to each other.

With reference to FIGS. 8 and 9, after welding lower edge 70 a of plate 70 to projecting appendage 25 of semifinished part 20, and upper edge 70 b of plate 70 to projecting appendage 46 of semifinished part 40, the method of producing blade 1 comprises trimming/machining, by milling or other material-removing machining operation, the excess material from the resulting part to obtain coupling root 2, airfoil-shaped oblong member 3, and coupling head 4 of blade 1 of the desired shape.

More specifically, during the step of removing the excess material from the part formed by butt-welding plate 70 to the two semifinished parts 20 and 40, the method of producing blade 1 comprises:

trimming/machining, by milling or other material-removing machining operation, excess metal off the two major faces of plate 70, including the joins between plate 70 and the two semifinished parts 20 and 40, to obtain airfoil-shaped oblong member 3; and/or trimming/machining, by milling or other material-removing machining operation, excess metal off the long side edges of plate 70, so as to form leading edge 3 a and trailing edge 3 b of airfoil-shaped oblong member 3; and/or trimming/machining, by milling or other material-removing machining operation, excess metal off the two semifinished parts 20 and 40 to obtain coupling root 2 and coupling head 4 of blade 1.

Once the above material-removing machining operations are completed, plate 70 forms center plate-like body 7 of airfoil-shaped oblong member 3; semifinished part 20 forms both coupling root 2 of blade 1 and lower connecting fin 5 of airfoil-shaped oblong member 3; and semifinished part 40 forms both coupling head 4 of blade 1 and upper connecting fin 6 of airfoil-shaped oblong member 3.

In addition to the above, and with reference to FIGS. 10, 11 and 12, in the example shown metal plate 70 of constant thickness, and of a three-dimensional shape over-approximating the shape of center plate-like body 7 of airfoil-shaped oblong member 3, is preferably formed from a large flat metal plate (not shown) of constant thickness, over-approximating the maximum thickness of center plate-like body 7 of airfoil-shaped oblong member 3 of blade 1, by a process comprising:

cutting, from said large flat metal plate (not shown), a plate piece 100 with a contour over-approximating the flat contour of center plate-like body 7 of airfoil-shaped oblong member 3 of blade 1; and

twisting and curving the plate piece 100 via press-forming (i.e. via forming with no noticeable reduction in the nominal thickness of the plate), so that the final three-dimensional shape of the centerline plane M of press-formed plate piece 100 substantially matches the three-dimensional shape of centerline plane P of center plate-like body 7 of airfoil-shaped oblong member 3 of blade 1, to obtain plate 70.

In the example shown, the flat metal plate is preferably 5 mm to 40 mm (millimeters) thick.

The particular structure of blade 1 and the method of producing blade 1 as described above have numerous advantages.

Firstly, the method of producing blade 1 allows the semifinished parts, eventually forming coupling root 2, airfoil-shaped oblong member 3 and coupling head 4 of blade 1, to be produced separately using production processes best suited to the three-dimensional shape and desired mechanical characteristics of each part.

Moreover, joining center plate-like body 7 to upper and lower connecting fins 5 and 6 along conjugate mating surfaces A₅, A₇, B₆ and B₇, which extend seamlessly from leading edge 3 a to trailing edge 3 b of airfoil-shaped oblong member 3 along a curved profile/path in the laying plane/centerline plane P of airfoil-shaped oblong member 3, enormously increases the rigidity and mechanical strength of blade 1, as compared with turbomachine blades with perfectly flat conjugate mating surfaces perpendicular to the laying plane/centerline plane of the airfoil.

Using a metal plate 70 of constant thickness over-approximating the maximum thickness of center plate-like body 7 of airfoil-shaped oblong member 3 of blade 1 furthermore drastically reduces the amount of excess material to be milled or otherwise machined off, and therefore also the time and cost this involves.

Last but not least, by welding the lower edge 70 a of plate 70 to projecting appendage 25 of semifinished part 20 (i.e. butt-welding the lower edge 7 a of center plate-like body 7 to ridge 5 a of lower connecting fin 5) along transverse reference line T_(a) formed by the locus of points in which the thickness of plate 70 over-approximates locally the maximum thickness of center plate-like body 7 of airfoil-shaped member 3 of blade 1, the amount of excess material to be removed from the weld area between the two parts is minimized, with all the advantages this affords in terms of mechanical strength of the weld between center plate-like body 7 and lower connecting fin 5 of coupling root 2.

Likewise, by welding the upper edge 70 b of plate 70 to projecting appendage 46 of semifinished part 40 (i.e. butt-welding the upper edge 7 b of center plate-like body 7 to ridge 6 a of upper connecting fin 6) along transverse reference line T_(b) formed by the locus of points in which the thickness of plate 70 over-approximates locally the maximum thickness of center plate-like body 7 of airfoil-shaped member 3 of blade 1, the amount of excess material to be removed from the weld area between the two parts is minimized, with all the advantages this affords in terms of mechanical strength of the weld between center plate-like body 7 and upper connecting fin 6 of coupling head 4.

Clearly, changes may be made to blade 1 and to the method of producing it, without, however, departing from the scope of the present disclosure.

For example, blade 1 may have no coupling head 4, thus simplifying the method of producing blade 1.

In other words, airfoil-shaped oblong member 3 may comprise lower connecting fin 5, which cantilevered projects from and is formed in one piece with coupling root 2; and center plate-like body 7, which in this case only forms an extension of lower connecting fin 5, and is butt-welded to, to form one piece with, lower connecting fin 5.

In another not shown variation, coupling head 4 of blade 1 may even be formed in one piece with center plate-like body 7.

With reference to FIG. 13, in a different embodiment of blade 1, airfoil-shaped oblong member 3 may have a closed weight-reducing inner cavity, the three-dimensional shape of which preferably, though not necessarily, substantially matches, to a smaller scale, the three-dimensional shape of airfoil-shaped oblong member 3 as a whole.

More specifically, in the example shown, airfoil-shaped oblong member 3 has, substantially in the center of one of its two major faces, at least one large hollow weight-reducing seat 8 of predetermined shape and preferably extending over 40% of the total area of the major face of airfoil-shaped oblong member 3; and furthermore comprises a cover plate 9, which is complementary in shape to the opening of hollow seat 8, and is fixed irremovably—preferably welded along its entire peripheral edge—to the major face of airfoil-shaped oblong member 3 in which hollow seat 8 is formed, so as to seal the opening of hollow seat 8 and form the sealed weight-reducing cavity of airfoil-shaped oblong member 3.

In which case, after removing the excess material from the part formed by butt-welding plate 70 to the two semifinished parts 20 and 40, the method of producing blade 1 comprises:

forming, on one of the two major faces of airfoil-shaped oblong member 3, by milling or other material-removing machining operation, a large hollow weight-reducing seat 8 of predetermined shape and preferably extending over 40% of the total area of the major face of airfoil-shaped oblong member 3;

cutting and curving a metal plate of suitable thickness, to form a cover plate 9, which is complementary in shape to the opening of hollow seat 8 formed previously on one of the two major faces of airfoil-shaped oblong member 3;

placing cover plate 9 over, so as to seal, the opening of hollow seat 8 and form the sealed weight-reducing cavity of airfoil-shaped oblong member 3; and

welding the peripheral edge of cover plate 9 to the peripheral edge of hollow seat 8 to secure cover plate 9 irremovably to the rest of airfoil-shaped oblong member 3.

Preferably, the shape of hollow seat 8 substantially matches, to a smaller scale, the three-dimensional shape of airfoil-shaped oblong member 3 as a whole.

If, in addition to center plate-like body 7 of airfoil-shaped oblong member 3, milling hollow weight-reducing seat 8 also involves one or both connecting fins 5 and 6, it will obviously be the nominal widths l_(a) and l_(b) of strip-shaped conjugate mating surfaces A₅, A₇ and B₆, B₇ that are substantially constant and over-approximate the maximum thickness of center plate-like body 7, at least along a center portion of a length equal to over 60% of the total length of strip-shaped mating surfaces A₅, A₇ and B₆, B₇. 

The invention claimed is:
 1. A method of producing a turbomachine blade comprising a metal coupling root; and a metal airfoil-shaped oblong member that includes a lower connecting fin cantilevered from the coupling root, and a main plate-like body forming an extension of said lower connecting fin; the method comprising: making a first semifinished metal part having a three-dimensional shape that over-approximates a shape of the coupling root of the blade, and including a projecting appendage having a three-dimensional shape that over-approximates a shape of the lower connecting fin; making a metal plate of substantially constant thickness over-approximating a maximum thickness of the main plate-like body, and having a three-dimensional shape that over-approximates a shape of the main plate-like body; removing a bottom portion of the plate located beyond a first transverse reference line having a curved profile in a laying plane/centerline plane of the plate, wherein said first transverse reference line has a substantially Ω (omega) shaped profile; removing a portion of a ridge of the projecting appendage of the first semifinished metal part located beyond a second transverse reference line which, in a laying plane/centerline plane of the projecting appendage, has a curved profile substantially identical to the curved profile of the first transverse reference line, wherein said second transverse reference line has a curved, substantially Ω (omega) shaped profile substantially identical to that of the first transverse reference line; placing a lower edge, corresponding to said first transverse reference line, of the plate on the ridge of the projecting appendage of said first semifinished metal part, so that the plate is aligned locally with the projecting appendage of the first semifinished metal part; butt-welding the lower edge of the plate to the projecting appendage of said first semifinished metal part to connect the plate and the first semifinished metal part rigidly to each other and produce a first resulting part; and trimming/machining excess material off the first resulting part to obtain the coupling root and airfoil-shaped oblong member of the blade.
 2. The method as claimed in claim 1, wherein said first transverse reference line is a locus of points in which a thickness of the plate over-approximates locally a maximum thickness of the main plate-like body of the airfoil-shaped oblong member; and said second transverse reference line is a locus of points in which a thickness of the projecting appendage of the first semifinished part over-approximates locally the maximum thickness of the main plate-like body of the airfoil-shaped oblong member.
 3. The method as claimed in claim 1, wherein trimming/machining the excess material off the resulting part comprises trimming/machining excess metal off of two faces of the plate.
 4. The method as claimed in claim 1, wherein trimming/machining the excess material off the resulting part comprises trimming/machining excess metal off of long side edges of the plate, so as to form a leading edge and a trailing edge of the airfoil-shaped oblong member.
 5. The method as claimed in claim 1, wherein said blade also comprises an upper coupling head, and the airfoil-shaped oblong member also comprises an upper connecting fin, which cantilevers from the coupling head towards the coupling root and connects to said main plate-like body; the method comprising: making a second semifinished metal part having a three-dimensional shape that over-approximates a shape of the coupling head of the blade, and including a projecting appendage having a three-dimensional shape that over-approximates a shape of the upper connecting fin; removing a top portion of the plate located beyond a third transverse reference line having a curved profile in the laying plane/centerline plane of the plate; removing a portion of a ridge of the projecting appendage of the second semifinished metal part located beyond a fourth transverse reference line which, in a laying plane/centerline plane of the projecting appendage of the second semifinished metal part, has a curved profile substantially identical to the curved profile of the third transverse reference line; placing an upper edge, corresponding to said third transverse reference line, of the plate on the ridge of the projecting appendage of said second semifinished metal part, so that the plate is aligned locally with the projecting appendage of the second semifinished metal part; butt-welding the upper edge of the plate to the projecting appendage of said second semifinished metal part to connect the plate and the projecting appendage of said second semifinished metal part rigidly to each other and form a second resulting part; and trimming/machining excess material off the second resulting part to also obtain the coupling head of the blade.
 6. The method as claimed in claim 5, wherein said third transverse reference line is a locus of points in which the thickness of the plate over-approximates locally the maximum thickness of the main plate-like body of the airfoil-shaped oblong member; and said fourth transverse reference line is a locus of points in which a thickness of the projecting appendage of the second semifinished metal part over-approximates locally the maximum thickness of the main plate-like body of the airfoil-shaped oblong member.
 7. The method as claimed in claim 5, wherein said third transverse reference line has a substantially Ω (omega) shaped profile; and said fourth transverse reference line has a curved, substantially Ω (omega) shaped profile substantially identical to that of the third transverse reference line.
 8. The method as claimed in claim 1, wherein a difference between the thickness of said metal plate and the maximum thickness of the main plate-like body of the airfoil-shaped oblong member is less than 2 millimeters.
 9. The method as claimed in claim 1, wherein making the metal plate of substantially constant thickness over-approximating the maximum thickness of the main plate-like body comprises: cutting, from a large flat metal plate of constant thickness over-approximating the maximum thickness of the main plate-like body of the airfoil-shaped oblong member of the blade, a plate piece with a contour over-approximating a flat contour of the main plate-like body of the airfoil-shaped oblong member; and twisting and curving the plate piece via press-forming, so that a final three-dimensional shape of a centerline plane of the plate piece substantially matches a three-dimensional shape of a centerline plane of the main plate-like body of the airfoil-shaped oblong member of the blade. 